Solar photovoltaic (PV) output will reduce a little when the modules reach high temperatures. As a rule of thumb, you can expect around 0.5% decrease in module output per degree centigrade temperature increase.
This does affect the design of roof arrays - as the modules need to be ventilated to prevent them getting excessively hot.
An array that is mounted above an existing roof (so a non-integrated array) will have a gap behind and at the sides, allowing air to circulate and heat to disperse.
Roof-integrated arrays, including PV slates or tiles but also other integrated systems, will not have this circulation, so the array will be prone to getting hotter and so being a bit less efficient.
There are measures that can help improve performance in integrated arrays. One option is 'counter-battening', which involves adding extra battens above the sarking but below the conventional battens (and at right angles to these). This would of course mean some extra roofing work, including removing all slates on that pitch - not just those being replaced with PV tiles.
Even with the extra ventilation, an integrated array will still suffer a little from lower performance than a non-integrated array - so it tends to only be used where it is essential for getting planning permission.
(This image is taken from the CAT book Choosing Solar Electricity).